Answer:This issue is because of the fact that if object already exists with the same SMTP inside Exchange then one is not allowed to create another object having same SMTP address. Though the issue could be faced on multiple instances in Exchange, however few of the scenarios are shared below for anyone’s convenience and shall help in some way.

Example for Mailbox: Error : 72300060 : Unable to create mailbox If you are trying to create a mailbox whose identical SMTP address has already been assigned to another mailbox from different organization.

Error : 72300210 : Mailbox user already exists If the mailbox already exists in that scenario too, you will get the following error while adding mailbox via HC panel.

Example for Distribution List: Error : 72300230 : List already exists. While trying to create Distribution List whose identical SMTP address is already associated with another DL in a separate organization as its Email Alias then in that case the following error is thrown while adding the Distribution List.

Similarly, for other Exchange objects where such an error is encountered, the problem can easily be identified and rectified by searching the Exchange object which is causing conflict due to same SMTP address assigned to it with the help of following Exchange PowerShell command:

If you want to know the exact OU name under which the object causing SMTP address conflict exists then you may run the following command which gives you all relevant details including OU for that particular Exchange object: